Photography / Exposure
ND Filter Calculator (Long Exposure Time)
Work out the shutter speed behind a neutral density filter, from ND2 to ND1000 and beyond or any number of stops, with a countdown timer for long exposures.
ND Filter Calculator (Long Exposure Time): An ND filter is named by how many times it cuts the light: ND8 passes an eighth, so the exposure must be 8 times longer, which is 3 stops. ND1000 cuts it a thousandfold, just under 10 stops, and turns 1/125 s into 8 s. The optical density printed on some filters is the logarithm of that factor, so ND 3.0 is the same as ND1000. Runs 100% locally in your browser with zero server file uploads.
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Each stop doubles the exposure: ND8 cuts the light 8 times, which is 3 stops, and ND1000 about 10. Very dark filters often need a little extra time, and some shift the colour; check the first frame and adjust.
A worked example
At the coast, the meter reads 1/60 s at f/11. Behind a 6-stop ND64 that becomes 1/60 × 64 ≈ 1 s, enough to blur the waves; behind an ND1000 it is about 17 s, which turns the sea smooth.
For the field of view of the lens you are using, see the crop factor calculator; to plan a series of exposures, the time-lapse calculator.
Getting the exposure right
Compose and focus before fitting a dark filter, because the viewfinder and autofocus may not see through it; switch to manual focus so the camera does not hunt. Cover the eyepiece on a DSLR during long exposures, since light entering it can fog the picture.
Use a tripod and a remote release or the self-timer. The depth of field calculator shows how much of the scene will be sharp at the aperture you chose.
How to use it
- Meter the scene without the filter and type that shutter speed, such as 1/125.
- Choose the filter, or enter its strength in stops.
- Set the new exposure on the camera, using Bulb and the timer beyond 30 seconds.
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Frequently asked questions
How long is 1/60 s with an ND1000?
About 17 seconds: 1/60 × 1000 = 16.7 s.
What is the difference between ND1000 and 10 stops?
Very little: 10 stops is a factor of 1024, ND1000 exactly 1000, so the times differ by about 2%.
Do I need to change the aperture or ISO?
No: keep them as they were when you metered, and change only the shutter speed.
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